A genetic system is being developed in the yeast Saccharomyces cerevisiae for the simultaneous study of chemical mutagen specificity by a variety of approaches. Other forward mutation and reverse mutation assays in fungal test systems have yielded data that are contradictory with regard to the predominating classes of molecular alterations induced by specific chemical mutagens. This system is designed to minimize differential effects of extrinsic and physiologic factors that influence mutagen specificity.